The invention relates to a retaining plate for the hand brake lever of a drum brake during initial assembly for generating a lasting initial tension of the Bowden cable.
Drum brakes for currently conventional cars are used both for the service brake and also to a particular extent for the parking brake. Drum brakes have been tried and tested for many decades.
They conventionally comprise two arc-shaped brake shoes, which are carried and supported on an anchor plate. A brake drum transmits the rotational movement of the wheel to be braked or the torque of a wheel to be prevented from rotating to the brake shoes. For this purpose, the brake shoes are moved towards the brake drum and brought into frictional engagement therewith. The driving force of the brake drum taken up by the brake shoes support them in the anchor plate.
In order to bring the brake shoes into frictional engagement with the brake drum, a wheel brake cylinder is provided for the service brake and contains two pistons, which upon loading by means of hydraulic pressure move the brake shoes towards the brake drum and press them against the brake drum. The generation of the hydraulic pressure occurs as a result of depressing the brake pedal, as a result of which in the master cylinder a piston presses hydraulic fluid through the brake line to the wheel brake cylinder.
For the parking brake a Bowden cable is provided, one end of which terminates in the passenger compartment, its sheath being supported on the bracket, and the hand brake cable is connected to the hand brake stem, wherein by pulling the hand brake stem a tensile force is exerted on the hand brake cable, and the other end of which terminates in the drum brake, the sheath being supported on the anchor plate, and the tensile force generated by the hand brake stem pulls by means of the hand brake cable on the hand brake lever, which therefore moves the two brake shoes apart from one another and presses them against the brake drum.
Depending on the nature of initiation of the actuating force and the nature of support of the friction force generated by the brake drum into the anchor plate, the ratio of the generated friction force to the applied actuating force may be controlled within wide limits such that a high self-boosting forms the one limit, at the price of a high tendency to lock and a large influence of the coefficient of friction between the friction surfaces, and a low ratio of generated friction force to applied actuating force forms the other limit, with the advantage of a low influence of the coefficient of friction on the generated friction force, i.e. a very stable brake.
Whereas previously power brakes, drum brakes with high self-boosting, were used in many cases, since the development and use of brake boosters more value has been placed on the stability of the brake.
For this reason nowadays a drum brake construction is mainly used, which comprises two brake shoes, of which the one, the primary shoe, has a leading action and the other, the secondary shoe, has a trailing action. Such a drum brake has a high efficiency combined with a low tendency to lock.
As time passes, the brake linings as well as the brake drum wear. In order to compensate this wear—for in the conventional hydraulically actuated brakes the actuating travel of the brake pedal and hence also the quantity of hydraulic fluid displaceable in the master cylinder are limited, as is the actuating travel of the hand brake stem and hence also the distance by which the hand brake cable may be pulled out of the sheath—these drum brakes are equipped with an automatic readjustment device.
Automatic readjustment devices for drum brakes very often operate by means of a variable-length strut, which is disposed between the two brake shoes and the length of which is increased in dependence upon the wear by means of a pawl and ratchet wheel mechanism. Such readjustment devices are known for example from the patent documents DE 2 508 614, and corresponding U.S. Pat. No. 3,963,100, both of which are incorporated by reference herein, FR 2 161 654 and U.S. Pat. No. 2,292,017.
These automatic readjustment devices generally only have the effect that the spacing between the friction surface of the brake drum and the friction surfaces of the brake shoes is as constant as possible. This relates both to the wear of the friction material and the wear of the brake drum. If the brake shoes are readjusted by means of the strut and the pawl and ratchet wheel mechanism, then the pistons in the wheel brake cylinder take up a new position, for example a helical compression spring disposed between the pistons presses the pistons apart from one another. Hydraulic fluid flows from the storage tank via the master cylinder and fills the enlarged space between the pistons. Also, by means of the strut the hand brake lever is readjusted. Both service brake and parking brake as such operate very reliably for years.
However, over the years the Bowden cable yields. A certain cable slack arises. This has the effect that, despite a correctly readjusted brake, upon pulling the hand brake stem the immobilizing effect occurs later, the hand brake stem has to be pulled a few teeth further before the original immobilizing effect arises.
This yielding was previously compensated by readjusting the Bowden cable during servicing. In the background art constructions are also known, which readjusted the wear and stretching of the cable pulls or linkages in the region of the operating lever by means of clamp-type locking mechanisms, wherein the clamping bodies are balls, or by means of a detent tooth system, German patent DE 444 779, or also by means of a ratchet mechanism disposed in the fulcrum of a double-armed lever, wherein at the first lever arm the actuating force generated by the vehicle driver was introduced and at the second lever arm this actuating force was passed on in a geared manner to the brake, wherein both lever arms were pulled by means of helical tension springs of differing strength in such a way about the fulcrum that the stronger helical tension spring pulled the first lever arm onto a stop and the second, weaker helical tension spring pulled the second lever arm back as far as the slack and the tooth system allowed, French patent FR 668 053.
A laborious task that has existed since introduction of the Bowden cable is also that the Bowden cable, after it has been fitted, connected to the hand brake stem in the passenger compartment and connected to the hand brake lever in the drum brake, has to be adjusted in such a way that on the one hand there is no cable slack, which entails lost travel for the hand brake stem, and on the other hand the brake shoes are however still not in frictional engagement with the brake drum.
The prior art includes a drum brake, German patent DE 696 200, in which in the released state of the brake an assembly pin locks the angle lever with the joint rod, wherein in this state the Bowden cable is adjusted. Afterwards, the assembly pin is removed.
From German patent DE 10 220 016, and corresponding U.S. Pat. No. 6,874,600B2, both of which are incorporated by reference herein, an assembly device for the cable pull of a parking brake is known, in which for eliminating the cable slack needed for easy assembly a retaining device is provided, which comprises a displacement sleeve and a fixing sleeve, wherein after assembly of the Bowden cable the displacement sleeve is displaced in the fixing sleeve and secured by means of a radially preloaded spring.
From the German patent DE 3 741 530 an automatic readjusting device for the cables of the parking brake is known, which by means of a torsion spring constantly endeavours to pivot the lever arms of the compensating yoke, which are mounted pivotably relative to one another, in such a way that the cables are tensioned. A ratchet mechanism in this case allows a rotational movement of the lever arms only in the direction of a re-tensioning of the cables.
From the German patent DE 4 023 047 a method of fitting a pre-assembled hand brake device is known. In this case, in order to allow the hand brake stem to be brought vertically through the opening of the base plate, the Bowden cable is fixed in a first position on the bracket by means of a resilient detent connection, and in this position the hand brake cables are also adjusted, and after introducing the hand brake stem into the passenger compartment the bracket is displaced by means of a piston-cylinder unit in such a way that the bracket may be screw-connected to the base plate. During this displacement of the bracket, the Bowden cable takes up its operating position.
Further documents that are concerned with cable stretching in the German patents DE 315 182, DE 1 030 982, DE 4 040 779, DE 19 734 572, and corresponding U.S. Pat. No. USRE 40760E1, both of which are incorporated by reference herein, and in DE-OS 3 103 708 and DE-OS 3 819 177.
From the German utility model DE 8 811 920 a mechanically and hydraulically actuable disc brake is known, in which for adjusting the initial cable tension and for compensating the cable stretching during initial assembly a spacer made of plastics material is introduced between a housing-fixed stop and a stop of the hand brake lever that interacts with said housing-fixed stop, so that the hand brake lever that is pressed by a torsion spring normally against the housing-fixed stop is supported for the initial assembly on this spacer. In this state, the hand brake cable is placed under initial tension until the spacer may be removed without any expenditure of force. The dimension of the thickness of the spacer is tuned to the yielding behaviour of the Bowden cable.
From the German utility model DE 9 209 637 and the European patent EP 579 178 equivalent thereto, a further development of the spacer known from the previously cited utility model DE 8 811 920 is described, to the effect that the spacer is also capable of taking up a tensile force. The hand brake cables may then be machine-adjusted more reliably without minor variations during tightening of the adjusting nuts of the Bowden cable, caused by friction coefficient differences of the thread and tolerances of the screws, leading to significant deviations of the spacing of the stop of the hand brake lever from the housing-fixed stop. This is achieved in that the Bowden cable during the initial tensioning generates a very steep rise in force and hence a steep rise in the torque of the screwdriver, with the result that the screwdriver switches off. After this basic setting, the spacer is broken by a single strong pull on the hand brake stem.
U.S. Pat. No. 6,241,051 B1 describes a brake cable connecting device for fastening a brake cable to a brake lever in a concealed manner in order to reduce the cost and weight of the braking device. The connecting device is formed by a U-shaped channel, which is provided at a free end of the brake lever, and a guide unit for automatically mounting the brake cable on the free end of the brake lever when the brake cable is pushed in the direction of the brake lever. The guide unit, which is manufactured separately from the brake lever, comprises a cable-receiving trough, which is bent into a U shape and mounted in the channel of the brake lever, a guide element for guiding the brake cable in the direction of the free end of the brake lever, and a ramp-shaped surface for deflecting the brake cable when the brake cable moves along its surface to generate a return force in the brake cable. The brake cable as a result of the return force snaps into place in the cable-receiving trough when its end passes the tip of the ramp-shaped surface.